The present invention is directed generally to medical devices, methods and systems. More specifically, the invention is directed to methods, devices and systems for analyzing calibration of laser ablation systems.
Ultraviolet and infrared laser-based systems and methods are known for enabling ophthalmological surgery on the external surface of the cornea in order to correct vision defects. These procedures generally employ an ultraviolet or infrared laser to remove a microscopic layer of an anterior stromal tissue from the cornea to alter its refractive power. In ultraviolet laser ablation procedures, the radiation ablates corneal tissue in a photodecomposition that does not cause thermal damage to adjacent and underlying tissue. Molecules at the irradiated surface are broken into smaller volatile fragments without substantially heating the remaining substrate; the mechanism of the ablation is photochemical, i.e., the direct breaking of intermolecular bonds. The ablation penetrates into the stroma of the cornea to change its contour for various purposes, such as correcting myopia, hyperopia, and astigmatism.
In such laser-based systems and methods, the irradiated flux density and exposure time of the cornea to the laser radiation are controlled, to provide a surface sculpting of the cornea to achieve a desired surface change in the cornea. To that end, ablation algorithms have been developed that determine the approximate energy density that must be applied to remove a certain depth of tissue from the cornea. At ultraviolet wavelengths, for example, a cumulative energy density of about 0.6 joule/cm2 to about 1 joule/cm2 will typically ablate corneal tissue to a depth of about one micron when applied in a series of pulses of about 100 to 400 millijoules/cm2. Accordingly, the ablation algorithms are tailored for each procedure depending on the amount and the shape of corneal tissue which will be removed to correct a particular individual's refractive error.
To properly use these laser ablation algorithms, the laser ablation system typically should be calibrated. Calibration of the laser system helps ensure removal of the intended shape and quantity of the corneal tissue so as to provide the desired shape and refractive power modification to the patient's cornea. In addition, it is usually desirable to test for acceptable levels of system performance. For example, such tests can help ensure that internal optics are aligned, that laser fluence is accurate, and the like.
Ablations of plastic test materials are often performed prior to excimer laser surgery to calibrate the energy density and ablation shape of the laser. During these tests, a lens is ablated into the test plastic, and the refractive power of the test lens is read by a standard lensometer. The reading from the lensometer is then entered back into the laser system so that the system can make appropriate calibration adjustments. The test lens may also be grossly evaluated under a magnifying glass, and test samples are sometimes sent to a laboratory for accurate evaluation to help determine beam homogeneity and quality.
Although known laser ablation calibration techniques are fairly effective, they still suffer from certain disadvantages. For example, delaying each surgery while obtaining accurate laboratory evaluations of a test lens may be impractical. Without such lab evaluations, when a user analyzes a test lens by merely examining the lens with the naked eye or using a conventional lensometer, it is almost impossible to detect high-order aberrations or artifacts on the test lens. Artifacts, generally, are localized/isolated ablation defects, often caused by inhomogeneity in the laser beam. Accurately estimating powers and shapes of the ablated test lens is also very subjective using only a conventional lensometer. On the other hand, requiring specialized test lens evaluation equipment, such as an interferometric surface profiler, at each site could add significantly to equipment costs and overall system complexity. Nonetheless, some information beyond an approximation of refractive power and a gross evaluation of the test lens would be helpful to improve the accuracy of regular calibrations, whether they are performed monthly, daily, or before each ablation procedure.
In light of the above, it would be desirable to provide improved methods, devices and systems for analyzing calibration of laser ablation systems. It would be particularly desirable if such improvements enhanced calibration accuracy without significantly increasing overall system costs and complexity. It would further be desirable if such improvements could provide quantifiable data which might be used in an automated calibration feedback and adjustment system. At least some of these objectives, will be achieved by the present invention.